The long-term objective of this project is a thorough understanding of the behavioral and neural mechanisms of sound localization in the auditory brainstem nuclei. The present application proposes to develop a behavioral preparation for testing sound localization in cats by training them to look at sound sources and to study the physiological responses of cells during this behavior. Previous studies of sound localization have chiefly centered on two areas: human or animal psychophysical work has established the important cues for sound localization and animal physiological work has shown that the nervous system is sensitive to these cues. This application is an effort to link these two approaches by combining animal psychophysics with physiology. There are five general aims. One is to study the psychophysics of sound localization in the cat using normal and spectrally shaped stimuli delivered in free field. Two is to develop a technique for delivering both free field and dichotic stimuli at the same time to determine how the cat hears the "virtual space" stimuli. Three is to determine whether cats exhibit the precedence effect and summing localization, two important psychophysical illusions that enable us to localize sounds in a reverberant environment. Four is to study the role of the cat's external ears, or pinna, in sound localization by measuring pinna movements during localization behavior and by studying the effects of paralyzing the pinna on the ability to localize sounds. Five uses this preparation for physiological studies of single cells in the superior and inferior colliculus in awake, behaving cats. Spatial hearing and sound localization are important basic functions of the auditory system: defects in binaural function in human patients can lead to considerable difficulty in detecting signals embedded in noise, such as understanding conversation in a noisy room, which is perhaps the most common complaint of the hearing-impaired and can lead to severe social withdrawal.